Chemical Reactions
What the exam expects
You need to write, balance, classify, and predict reactions, and to apply stoichiometry. Core concepts: conservation of mass (balancing equations); reaction types (synthesis, decomposition, single replacement, double replacement, combustion, acid-base neutralization, redox); solubility and precipitation; net ionic equations; the mole concept and molar mass; limiting reactants and percent yield; and solution concentration (molarity).
Traps to watch for
- Unbalanced equations - always balance before doing stoichiometry.
- Spectator ions - in net ionic equations, cancel ions that appear unchanged on both sides.
- Limiting reactant - the one that produces the least product, not necessarily the smallest mass.
- Mole ratio errors - coefficients, not subscripts, give the ratio.
- Molarity = moles solute / liters of solution (not solvent).
- Redox identification - track oxidation numbers; the substance oxidized is the reducing agent.
Rule of thumb
For stoichiometry: convert to moles → use mole ratio → convert to target units. Always check which reactant runs out first.
Worked example
Balance and use: 2 H₂ + O₂ → 2 H₂O. If 4.0 mol H₂ reacts with 3.0 mol O₂, H₂ is limiting (needs only 2.0 mol O₂). Moles of water = 4.0 mol H₂ × (2 mol H₂O / 2 mol H₂) = 4.0 mol H₂O, mass ≈ 4.0 × 18.0 = 72 g. Excess O₂ = 3.0 − 2.0 = 1.0 mol remains.